1. Field of the Invention
This invention relates to the field of heat exchangers. More specifically, this invention relates to heat exchangers for concentrating solids by the evaporation of liquids.
2. Discussion of the Prior Art
There are a variety of processes in which a solids containing stream is concentrated by the evaporation of liquids. In particular, multiple exchangers that operate as evaporators are used in desalination and the sugar industry to concentrate refined sugars into solid form, and have potential use in the mining industry for the concentration of ores in leaching operations and generally in the concentration of toxic materials from aqueous or other streams. One such evaporation step is in the treatment of black liquor from a papermaking process and more particularly relates to a process for concentrating black liquor from a Kraft process to a high solids content to enhance the usefulness of the material as fuel for soda recovery boilers.
The Kraft process is used extensively for the separation of clean cellulose fibers, known collectively as pulp, from various wood sources by dissolution of these fibers from small pieces of wood in a hot, strongly alkaline solution inside a vessel known as a digester. Typically, the liquid entering the digester is an aqueous solution consisting of a mixture of sodium sulfite (Na.sub.2 S), and sodium hydroxide (NaOH). The liquid exiting the digester consists of a weaker alkaline solution of approximately 15 wt. % total solids in an aqueous solution known as black liquor. This liquid consists of the inorganic materials fed to the digester plus the organic materials found in wood known collectively as lignin. The Kraft process continues by concentrating this dilute black liquor solution from a concentration of less than 20 wt. % to concentrations above 50% where the lignin can be consumed by combustion and the inorganics can be recycled.
The concentration of black liquor in the Kraft process is achieved by evaporation of water from the solution. The evaporation of this water is usually accomplished by heat transfer from steam on the shell side to the black liquor flowing upwardly on the tube side across a number of interconnected series flow vertical tubular heat exchangers. Such a series of heat exchangers is known as a multi effect evaporator (MEE), with each such heat exchanger known as an effect. MEE's are typically staged in pressures and associated temperatures. Typically the black liquor flow is from the lowest pressure to the highest, with concentration by boiling occurring in each effect. The evaporated water in each effect then can serve as the heat source for the next effect, operating at lower pressure and black liquor solids concentration. Typically, the evaporators operate as vertical thermosyphon exchangers.
Proper heat recovery in the concentration of solids promotes operational efficiency in the pulp and paper industry. The supply of steam to the effects, represents a direct and substantial operating expense to the pulp production process. Any increase in the heat transfer efficiency of the effects will directly reduce the operating cost of the process by lowering net steam consumption per pound of total water removed.
The high solids concentration of the black liquor presents a difficult challenge for the heat exchange service. Bare tube exchangers are prone to fouling. Regular cleaning is necessary to maintain suitable heat transfer performance.
It is known in the art that the surface of heat exchanger tubes can be treated to allow nucleate boiling. Such nucleate boiling surfaces can promote dramatic increases in the boiling film coefficients that are associated with heat transfer tubes in a boiling heat exchange zone. Such enhanced boiling surfaces for heat exchange tubes are discussed in U.S. Pat. Nos. 3,384,154; 3,821,018; 4,064,914; 4,060,125; 3,906,604; 4,216,826 and 3,454,081. Such surfaces have been known to provide benefits to a variety of processes. For example, a significant improvement in the refrigeration of an alkylation effluent by the use of an enhanced boiling surface is taught in U.S. Pat. No. 4,769,511. Such nucleate boiling surfaces have been known to increase boiling film heat transfer coefficients by a factor of 10 or more. It is also known that the use of one or more effects with an enhanced boiling service has improved the operation of multi-effect evaporators in the evaporation of water from ethylene glycol where the liquid streams are free of dissolved or undissolved solids.
Since liquid in a heat exchange tube begins to boil not when its bulk temperature reaches the boiling temperature, but when a tube wall temperature reaches the boiling temperature, dissolved or undissolved solids from the liquid can concentrate in localized areas near the tube wall. A nucleate boiling surface greatly increases localized boiling and therefore it was expected that such a surface would increase the concentration of solids on such surfaces while also locally drying micropores of the nucleate boiling surface. Thus, the combination of localized solids concentration and drying at the tube wall would lead one to anticipate fouling on the enhanced nucleate boiling surface and, after relatively short periods of time, reduce overall heat transfer relative to a bare tube wall.